How and when does clipping of RGB values happen?
This is result of Diffuse shader code implementation. Probably somewhere in the code is:

if R + G + B = 0:    # black
    renderSurfaceBlack()


To back this up I tested with this node setup:



And a cube on grey ground:



You can see red cube with red bounce light (color bleed) and red-complementary bounce light (cyan) under a anti-red (cyan) cube but which is rendered black (explained later).

When some channel is more than 1 the bounce light will look like emission (the light gets energy on bounces from the cube - energy is not preserved) and the surface will reflect redder red than it should (it will clip at 1,0,0 rendered red):



Till now all makes some sense. When the channel is below -1 actual weird stuff happens:



Try to render this, the sampling process is very wild in the start and looks like a burning fire under a piece of coal:)

When the sum of channels is not 0, all the negative values are replaced with 0 and that surface color is rendered.. You will still see the effect of negative channels in bounce light.



To sum this diffuse shader behaviour up:

When the channel sum is 0 the surface color is black.

When the channel sum is not 0, the surface color is same as with all negative channels equal to 0.